Local anesthetics (LAs) are widely employed to achieve tissue infiltration, peripheral and regional anesthesia, and neuraxial blockades. Despite their well-established toxic dose limits, these agents continue to pose a substantial risk of morbidity and mortality due to local anesthetic toxicity and overdose.
For example, LAs and epinephrine account for a large proportion of medication errors resulting in adverse patient outcomes due to drug dosing miscalculations or errors converting between units. Dosage calculations vary by patient weight as well as by pharmacokinetics and pharmacodynamics of individual LA formulations. Further, non-standard units, additives (epinephrine), and varying concentrations among LAs complicate correct dosage derivations.
In an effort to curb calculation errors and avert LA toxicity, Williams and Walker derived a helpful nomogram1 to calculate the maximum, weight-based volume of commonly used LAs (lidocaine, prilocaine, bupivacaine, and ropivacaine). This nomogram was validated against a calculator in the original article. Please note that while this nomogram may aid in dosage verification, there is no substitute for a second, independent derivation of the total maximum dose using a different method, as an additional safeguard to prevent dosage error.
Local anesthetic toxicity presentation
LA toxicity presents clinically as a constellation of symptoms including, but not limited to, tinnitus, circumoral tingling, metallic taste, and dizziness. Severe manifestations include altered mentation, arrhythmias, and cardiovascular collapse. Management is predicated upon stopping the offending agent, providing supportive measures, and administering weight-based intravenous 20% lipid emulsion. The authors, Williams and Walker, derived a separate nomogram to guide treatment by calculating the appropriate weight-based lipid therapy, specifying the initial bolus amount, infusion rate, and total maximum dose of lipid emulsion.
Both the toxicity and lipid emulsion nomograms are displayed in this Paucis Verbis card.
Ideal Body Weight (IBW) Calculation
The Devine formulation is the most commonly accepted calculation (most applicable for people at least 60 inches, or 5 feet, tall):
- IBW for men (kg) = 50 + 2.3 * (Height (in)-60)
- IBW for women (kg) = 45.5 + 2.3 * (Height (in)-60)
See the MDCalc calculator for IBW.
Great writeup, David. When this nomogram first came out, I was thrilled to see a much less complicated and more visual process to calculate LA toxicity levels. That being said, you are correct that a double-check approach to calculating is always a good idea. Medication calculations are definitely a source of much error and potential patient harm. I\'ll be sure to be using this PV card on shift.
One thing that might help is to also include an ideal body weight calculator, since this is one of the two required items on the nomogram (right column). This could at least be in the blog post since there\'s no space on the card. Would include a link, if you can find one.
Dr. Lin, I really like the idea to provide a link to an online IBW calculator and think its an important addition to this post. Although UptoDate\'s calculator would be my preferred link, this service is only accessible for those with a subscription (as is Epocrates\' calculator). MDCalc is my recommendation as its free, easy to use, and is a well-established website that is likely to be functioning for years to come. This calculator employs the Devine method, which seems to be the most widely used formulation for calculating IBW.
Expert Peer Review
David, this is a clinically useful tool that will be helpful for any practitioner performing procedures. Not having to recall the toxic dose thresholds for various local anesthetics from memory will potentially serve to reduce inadvertent overdoses. I particularly like the additional tool providing dosing recommendations for IV lipid emulsion if needed.
Your point about ensuring an additional double check of the dose is noteworthy, although the authors of the nomogram did validate it against calculated doses.
Finally, it\'s important to note that this nomogram does not apply for anesthetics administered via the IV or nebulized route (such as lidocaine). An ALiEM post from 2013 outlines safe dosing of nebulized lidocaine.
Expert Peer Review
David, to echo Michelle and Bryan, a great writeup and a very useful tool indeed.
I did have a practical question. The authors in the paper used the above chart printed on laminated A4 paper. Many (if not all) clinicians today rely on tablets and smartphones for storing reference materials and PV cards. I haven\'t had a chance to do this, but I wonder how much error might be involved in drawing a line on an iPhone screen (or using an edge of a note card as a ruler onscreen) vs A4 paper to determine the maximum volume of local anesthetics from the above PV card. Making an app using the above algorithm could be very useful in automating the calculations.
You raise a very intriguing question. In the transition from paper to digital media, this is somewhat uncharted territory. Regarding the image appearance on a digital screen, there should be no effect upon the derived LA volumes as screen distortions of the image are proportional to all axes. Stated another way, as long as the image itself is not deliberately altered to manipulate the position of the central axis relative to the others, the nomogram\'s digital projection is functionally equivalent to the paper version. Now, the differences in how users interface with each of the two media, and the relative error rates between digital and paper nomograms are unknown. I was unable to uncover any studies that have addressed this issue. Unfortunately, any difference that may exist will remain unknown until an investigation is undertaken or, perhaps more likely, until the nomogram is converted into an app, and thus obviating the issue.